The invention relates to an architecture for a hydraulic braking system adapted to aircraft having landing gear with braked wheels.
In general, an airplane is fitted with a main hydraulic circuit, said hydraulic circuit being arranged to feed the braking actuators of wheels carried by main landing gear units, said actuators being in the form of brakes each comprising a first series of xe2x80x9crotorxe2x80x9d disks constrained to rotate with the wheels and associated with a second series of xe2x80x9cstatorxe2x80x9d disks that are prevented from rotating, the disks in the two series alternating along the axis of rotation of a wheel and being pressed against one another by pistons mounted to slide in a hydraulic ring and actuated by means of hydraulic fluid under pressure delivered by the main hydraulic circuit of the aircraft. This pressure of the disks against one another then generates friction because the rotor disks are rotating with the wheel while the stator disks are prevented from rotating. This dissipates the kinetic energy of the aircraft in the form of the heat that is generated by the friction, thus slowing down the aircraft.
Braking is a critical function for an aircraft, and if braking fails completely, e.g. on landing, then there is an unacceptably high likelihood of passenger lives being at risk, not to mention the possibility of the aircraft itself being damaged. Furthermore, safety requirements lead to aircraft systems being designed so as to ensure that mere breakdown of any one system (e.g. the main hydraulic circuit) cannot lead to a catastrophe.
Thus, as a general rule, aircraft are fitted with an emergency hydraulic circuit whose hydraulic fluid is applied to the brakes in the event of the main hydraulic circuit failing. There are two approaches to brake design. In a first variant the brakes have dual cavities, i.e. their rings carry two series of pistons, a first series being actuated by the main hydraulic circuit, while the second series is actuated by the emergency hydraulic circuit. The two circuits are thus kept apart all the way to the final actuators. In a second variant, each brake has a single cavity only, i.e. only one series of pistons which can be fed selectively from one or other of the circuits via a shuttle valve which is generally situated in the wheel well and which delivers hydraulic fluid to the brake from whichever circuit is under the greater pressure. The advantage of this configuration is that the brake rings are simplified, as is the hydraulic pipework that extends along the landing gear itself, since a single pipe per brake then suffices.
Given the size and weight of the commercial aircraft presently under consideration, manufacturers have been constrained to consider using large numbers of main landing gear units, for example two wing units and one or two fuselage units.
This increase in the number of landing gear units leads to a corresponding increase in the amount of pipework required for the two braking circuits, particularly since each wing or fuselage landing gear unit is expected to be fitted with at least six wheels. This increase number of pipes and associated increase in pipework length due to the large size of the aircraft leads to an economically unacceptable penalty in the weight breakdown of the aircraft.
In order to simplify and lighten the braking system of such an aircraft, the invention proposes an architecture for a hydraulic braking system suitable for an aircraft of the type having at least one group of main landing gear units, each landing gear unit comprising a determined number of wheels each provided with a hydraulically actuated brake, the or each landing gear group being associated with a hydraulic circuit provided with hydraulic equipment and adapted to deliver hydraulic fluid under pressure to all of the brakes of the landing gear group, the hydraulic fluid being pressurized by at least one aircraft pressure generator system associated with an aircraft hydraulic fluid supply. According to the invention, accumulators are connected on the or each circuit in sufficient number for each accumulator to feed two pairs of brakes, each pair of brakes being mounted on a distinct landing gear unit, and an electrically-driven pump being arranged to maintain a predetermined pressure level in all of the accumulators of the circuit in question.
Thus, failure of any one circuit affects only a single landing gear group, and the brakes carried by the landing gear in another group continue to be fed normally by the other circuit. In the event of one of the circuits failing, e.g. due to pipework breaking or to the pressure generator system failing, then the accumulators, assisted where appropriate by the electrically-driven pump, take over to maintain sufficient pressure to provide braking that is acceptable for passenger safety. If these means also should fail, the aircraft would still retain braking ability on the other landing gear group, which is not possible with prior architectures. In addition, this architecture enables the hydraulic pipework to be simplified considerably, since there is no need to take pipes from both circuits to each landing gear unit. This architecture also makes it possible to make use of single-cavity brakes, which are lighter in weight and less complex than dual-cavity brakes.
Naturally, the landing gear groups should be organized in symmetrical manner so that total failure of any one circuit will not cause the remaining braking capacity to be asymmetrical, since that would make the pilot""s work much more difficult. For example, one landing gear group could be a wing group and another could be a fuselage group, each group having its own feed circuit. It will be understood that under such circumstances the architecture would not be implemented in aircraft having only two wing landing gear units each organized as a separate group, since under such circumstances the failure of one circuit would make braking highly asymmetrical which would be difficult to control.
In an emergency, the pressure available for the brakes comes from the associated accumulator whose capacity must be sufficient to enable the brakes connected to the corresponding circuit to be operated. Under normal circumstances, pressure is maintained in the accumulator by the pressure generator system of the hydraulic circuit. In an emergency, this pressure is maintained by the electrically-driven pump. In addition, the accumulator can be used for the parking brake, i.e. for preventing the aircraft from moving when it is stationary and its engines are not running. Furthermore, since the electrically-driven pump is driven by a motor that is electric, it is not sensitive, a priori, to hydraulic breakdowns.
The failure of any one accumulator involves only two pairs of wheels, each pair being situated on a different landing gear unit, which means that the aircraft retains significant braking capacity, since only four brakes are lost out of a total of about twenty. It is thus possible to clear an aircraft for takeoff even if it has a faulty accumulator.
For safety reasons, each accumulator is advantageously fitted with an overpressure relief valve.
Thus, in the event of pressure in an accumulator becoming excessive, the valve opens and allows a certain quantity of hydraulic fluid to escape into the aircraft""s fluid supply, thereby allowing the pressure to drop down to a safety threshold of the accumulator.
Also for safety reasons, the accumulator is fitted with a check valve on the line connecting it to the circuit so as to prevent it from discharging into the circuit in the event of the pressure in the circuit falling.
Still for safety reasons, provision is made for the electrically-driven pump to be associated with its own supply of hydraulic fluid.
Thus, if the aircraft""s fluid supply is not available, it is still possible to make use of the pump""s fluid supply for braking purposes.
Finally, in an emergency, in order to avoid emptying the pump""s fluid supply into the aircraft""s supply, the hydraulic circuit advantageously includes a general check valve upstream from the pump, together with a return selector uniting all of the return lines from all of the hydraulic equipment situated downstream from the check valve and directing the return flow of hydraulic fluid either to the aircraft""s supply or else to the pump""s supply.
Thus, by controlling the return selector so as to direct the return hydraulic fluid flow to the pump""s supply, a closed circuit is established which does not run any risk of losing fluid into the aircraft""s supply.
Each piece of equipment is advantageously fitted with a check valve on its own return line so that in the event of one of the return lines being broken, the pump""s fluid supply is not emptied out via the broken pipework.
This kind of aircraft is expected to fly for very long periods of time, of the order of 15 to 20 hours. Over such a length of time, internal leaks in various pieces of equipment can become non-negligible and can compromise the capacity of the electrically-driven pump to maintain the required pressure level in the portion of the circuit that is isolated by the upstream check valve. In particular, certain pieces of equipment such as proportional control valves (described below) are known to give rise to non-negligible return flow rates. The above disposition ensures that the supply of fluid does not empty out via a broken return line of a piece of hydraulic equipment, given that in practice the return lines are all connected together.
Advantageously, provision is made to arrange at least one braking selector downstream from at least one accumulator so as to allow the hydraulic fluid under pressure to reach the brakes associated with said accumulator, or else prevent it from reaching those brakes.
This selector serves to ensure that the brakes are not operated in untimely manner while the aircraft is in flight, and also to isolate a circuit in the event of pipework breaking downstream from the selector, so as to ensure that the hydraulic fluid supply of the aircraft is not emptied.
In order to apply braking, at least one proportional control valve is provided downstream from at least one braking selector.
The function of the proportional control valve is to modulate the pressure that is actually applied to the pistons of the ring of the associated brake(s), with this modulation being controlled by a braking controller that optimizes pressure as a function of information given by the pilot or by the onboard computer of the aircraft, and as a function of feedback information such as wheel rotation speed, its angular acceleration, or indeed the pressure actually available at the brakes, in application of a predetermined algorithm. The braking controller generates signals for controlling the proportional control valves which then respond as a function of said signals to apply the required pressure to the associated brakes.
In order to provide the parking brake function, it is advantageous for at least one parking selector to be provided downstream from at least one accumulator.
Thus, the pressure of the accumulators is transmitted directly to the corresponding brakes, without said pressure being transmitted via the line that includes the braking selector and/or the proportional control valve(s).
Advantageously, a shuttle valve is placed on the brake feed line connecting it either to the associated proportional control valve or else to the parking selector from which it depends.
In order to monitor the system as a whole, pressure sensors are advantageously provided on the circuit to measure the pressure that exists in the circuit at the outlet from each accumulator.
By means of these sensors, it is possible at all times to monitor the state of the circuit and to trigger alarms, whenever necessary.
To summarize the invention, on tracing the hydraulic circuit line of the aircraft conveying hydraulic fluid under pressure from the pressure generator system, there will be found in succession: the general check valve, and then the connection from the electrically-driven pump to the circuit. The line then splits into as many lines as there are accumulators, or, which amounts to the same, as many lines as there are twice two pairs of brakes. Tracing any one of these lines, there can be found a check valve preventing the accumulator from discharging into the general circuit in the event of pressure in that circuit falling, and then there is the accumulator. At the outlet from each accumulator, and downstream from the above-mentioned check valve, there can be found a line that is opened or closed by the braking selector. At the outlet from this selector, the line is duplicated to feed the two pairs of brakes connected thereto, each pair being mounted on a different landing gear unit. The line is then further duplicated to feed the two proportional control valves for the two brakes in the pair concerned, and finally the line terminates at the brake associated with the control valve via the shuttle valve.
Fluid can also be transmitted directly to a brake without passing via the line that includes the braking selector and the associated proportional control valve. The line coming from the accumulator associated with the brake in question is duplicated so as to be also connected to the parking selector which is in turn directly connected to the brake via the above-mentioned shuttle valve.
Finally, the return lines from each proportional control valve, brake selector, or parking selector are taken to a return selector via check valves, said return selector directing the return flow of hydraulic fluid either to the aircraft""s fluid supply, or else to the electrically-driven pump""s fluid supply.